Dual concentric drill pipe systems are particularly useful for air lift drill techniques, especially air lift reverse circulation drilling. A system for reverse circulation dual tube drilling is disclosed in Henderson U.S. Pat. No. 3,208.539.
In conventional circulation, single tube drilling systems, drilling fluid (e.g., mud or water) is pumped down the drill pipe to the bit, and returns to the surface in the annular space between the drill pipe and the hole wall (the hole annulus). An accepted technique involves the injection of compressed air into the drilling fluid, usually by means of an auxiliary air line, to assist in lifting the cuttings from the bottom of the hole to the surface.
The use of dual concentric drill pipe can greatly expand the possible applications of the air lift technique. In a dual tube system, such as shown in the above mentioned Henderson patent, an inner tube is disposed concentrically within an outer tube, thus defining a continuous annular flow passage between the two tubes (the pipe annulus or annular passageway) and a continuous flow passage through the inner tube (the central passageway). Air lift techniques can be used in such systems in a variety of ways. For example, a heavy drilling fluid such as mud or water can be pumped or allowed to pass down the hole through the hole annulus to the bit. At the same time, compressed air is pumped down the pipe annulus. The cuttings and drilling fluid then pass upward to the surface through the central passageway inside the inner tube. The compressed air in the pipe annulus is either injected into the inner tube at a location above the bit or passes around the bit and into the inner tube, and assists in lifting the cuttings upward. As another example, it might be desired to drill using only air, with no circulation in the hole annulus, such as in water bearing formations. In such an application, air would again be pumped down the pipe annulus, around the bit and into the inner tube. This air assists in the cutting process and would also serve to lift the cuttings and formation water out of the hole into the inner tube.
In such applications, it has been found that pump efficiency and lifting capacity may be greatly enhanced by introducing compressed air in one or more stages along the length of the drill string. It has also been found that the most efficient and desirable method for injection of air requires a system which minimizes turbulence and which causes the air to enter the water or mud stream in the inner pipe in extremely small bubles.